Polymeric resins are extrusion coated through dies to produce a polymeric coating. In producing photographic paper, a polymeric resin, such as polyethylene for example, is extrusion coated through dies onto the photographic paper substrate. The internal cavity of the die is normally of constant cross section across the coating width thereby enabling manufacture of various coating widths using a single die. Typically, coating width changes are made by adjusting external deckles which restrict the flow of resin through the slot opening. A common manufacturing problem resulting from the constant cross section internal and external deckles is non-uniform coating thickness at the edges of the coated material. The non-uniformity of thickness can be caused by a number of factors such as non-uniform cavity pressure, non-uniform molten resin temperature, or non-uniform Theological properties.
Prior efforts at controlling edge contours have focused on internal deckles with external width adjustment capability to improve edge bead thickness and thickness uniformity of the extruded layer near the edge, internal deckles with multiple stepped inserts and improved seals, and internal deckles with flow control obtained by changing the temperature of the insert or by providing separate flow channels to introduce a different resin material into the edge region. Such efforts are disclosed in U.S. Pat. Nos. 6,106,268; 5,830,391; 5,582,850; 5,575,85; 5,505,609; 5,395,231; 4,659,302; 4,283,168; 3,694,132 and 2,982,995. These efforts at controlling the flow of resin near the edge try to mimic a clothes hanger shaped internal die cavity (tapered from center to edge) that is normally limited to constant width applications. The clothes hanger cavities are designed for specific flow conditions and resin rheological properties. In addition to multiple width limitations, resin flow rates and properties limit use of clothes hanger dies.
Constant cross section cavity extrusion dies are commonly constructed with adjustable lip sections to accommodate the cross width pressure distribution and resulting non-uniform resin flow. Mechanical design constraints have limited cross width bolt adjustment intervals to approximately one inch. Structural requirements for the die deformation require substantial adjustable lip cross sections. Both of these items, bolt spacing and stiff sections, limit the usefulness of external die bolts to adequately provide the fine cross width resolution needed to control coating thickness at the edges.
It is desirable to have an internal deckle closely fitted in the die cavity with an externally adjustable cross width. It is also desirable to control the flow of resin in a localized region near each edge of the die and to change resin flow and coating width independently of each other. While obtaining finer flow control, it is also desirable to reduce resin stagnation and the resultant resin physical property degradation.